The 4th edition of Linux Device Drivers is not a file you download. It exists in the commits of torvalds/linux , the pull requests of GitHub, and the Stack Overflow answers of kernel maintainers.
Do not look for a PDF. Clone the kernel docs: Linux Device Drivers 4th Edition Pdf Github
Why? The kernel moves too fast. By the time a book about Linux device drivers is printed, several subsystems have already changed their APIs. Maintaining a printed 4th edition would be a Sisyphean task. The 4th edition of Linux Device Drivers is